hi there im a bit further away than normal but have worked around there and the st.albans area. No matter what the amount of work that needed doing was i wouldnt say it would cost anymore that £5,500. If you could give me any details of what the current boiler is and where you'd like the new one siting i could give you a more accurate idea. also where is the gas main and is it possible to run to the new boiler, hope this helps all the best
kev